This invention relates to a method and apparatus for increasing contrast between an object and space to determine the contours of the object when photographing a transparent object with a television camera.
It is well known to use television cameras to automatically measure the contour of a variety of opaque objects in various manufcturing industries. Typically, such an object will be illuminated from behind the image will be photographed by a television camera. The resultant image is then stored and compared to a reference image. If the deviation between the manufactured object and the reference image is greater than a predetermined tolerance, then necessary adjustments are made in the manufacturing equipment and the faulty object sorted out or discarded. These systems are conventional and well known to those of ordinary skill in the art, see for instance K. Melchior and G. Pavel "Automatisieren von Prufvorgangen mit bildverarbeitenden Sensoren", Technisches Messen, 50, 1983, p. 185-190 and 225-231.
Typically, this method works very well and accurately when the objects being scanned or photographed by the television camera are completely or substantially completely opaque, such as metal objects, since the boundary between transmitted light region and non-transmitted light is readily apparent. However, a problem arises during automatic measurement of the contour of transparent objects such as glass objects because the glass objects appear dark only at specific edge regions or at other points of marked curvature due to the transparency of the glass. It is therefore difficult or impossible, with conventional cameras operating in the visible and near infrared region of the spectrum, to determine the contour of the glass object automatically by conventional line scanning techniques. This presents serious problems because it is important to determine the contour of these objects so that the geometric dimensions of the glass objects can be determined automatically and compared with a prototype during manufacture of the glass objects or for establishing the position of a glass object relative to the border of the image. This determination of the relative position forms the basis for automatic image recognition for controlling manufacturing robots involved in the manufcturing process.
It is important to note that the term "transparent" as used above and hereinafter when referring to an object scanned or photographed is intended to mean capable of permitting passage of radiation or particles, but not necessarily permitting passage of all radiation and particles. Thus, a "transparent" medium or object in this case is one which has the property of transmitting rays of light or other electromagnetic radiation in a way that the human eye may see through the medium distinctly but which does not transmit all light wavelengths or electromagnetic radiation. One example is oxide glass normally used in industry which is transparent only to electromagnetic radiation having a wavelength of about 350 nm-4.5 .mu.m.